Dry type transformers are widely used, e.g., in the forms of high voltage distribution transformers, power supply transformers, welder transformers, mine transformers, and the like. Such transformers are constructed with sheet-like layer and barrier insulation. In the past, special papers, such as cyanoethylated kraft wood pulp, have been used as transformer insulation, because of their good resistance to thermal degradation and improved thermal stability. More recently, high temperature resistant polymer fibers, e.g., the polyaramids, have been made into papers for this purpose, e.g., the NOMEX paper available from DuPont and Co.
Transformers of the above-mentioned types have a natural tendency to show a temperature rise during operation. For example, measurements of internal temperatures can show an increase from room temperature to about 180.degree.-200.degree. C. after 4.5 hours of continuous operation if polyaramid paper is used. This puts design limits on size and has an effect on useful life, since it is a rule-of-thumb that a 10.degree. C. rise in operating temperature cuts transformer life by 50%.
It is clearly desirable to provide a simple means to reduce energy waste without changing the design of the transformer, and to lengthen its life. Such an object is achieved by the present invention.
By simply replacing the present insulation in dry type transformers with woven glass cloth saturated with a lightly-plasticized alkyd-type resin, the transformer can be upgraded in rating without increasing its size. Or the transformer can be redesigned to a smaller size without lowering its rating. Reductions in size will also result in significant savings in the cost of steel and copper.
It is believed that the new insulation is superior because of its vastly higher thermal conductivity in comparison with conventional materials. This permits the transformer to run cooler. There is no tendency to liberate hydrogen cyanide through combustion as is seen with some of the conventional materials. And because of its thermal conductivity, heat will be led away from insulating areas reducing chances of hot spots and runaway conditions.
The insulation of this invention can be used alone to build cool running dry type transformers, or it can be used with approved solvent-containing or solventless saturating resins of the polyester or epoxy type for superior insulation properties. These latter resins can also serve to waterproof and encapsulate the system through potting, dipping or vacuum pressure impregnation processes. Cure time is usually 2-4 hours at 150.degree. C. depending on the size of the unit treated.
In otherwise identical transformers, the insulation of the present invention will achieve an operating temperature differential of as much as 33.degree. C. lower than the commonly used materials.